1. Field of the Invention
The present invention is directed to strain sensors, and more particularly, a solid-state sensor that detects strain based on the electrostrictive response of the corresponding dielectric.
2. Description of Related Art
Strain gauges or sensors have been employed in a wide variety of applications. Conventional strain sensors are typically used for measuring the expansion and/or contraction of an object under stress. A common type of strain sensor comprises a resistive transducer. Other types of strain sensors include air-gap capacitive sensors, piezo resistors, silicon strain gauges, piezoelectric devices such as lead zirconium-titanate (PZT), capacitors formed of inter-digitative fingers that simulate adjacent parallel capacitors, conductive elastomer resistive strain gauges, as well as others.
Resistive strain sensors generate an electric output that is proportional to the amount the object being measured is deformed under strain. In one type of resistive strain sensor, the sensor is made of a metal foil or metal wire that is mounted on a substrate. In operation, the resistance of the wire changes with expansion or contraction of the object it is mounted on in a predetermined direction. Such a sensor requires either a DC or an AC excitation voltage to generate a strain signal. In addition, auxiliary equipment (for example, connecting the sensor in a differential arrangement such as in a wheatstone bridge circuit); typically must be provided to accurately determine the amount of strain.
Capacitance strain gauges, such as those shown in FIGS. 1A and 1B, depend on geometric features of the gauge to measure strain. In FIG. 1A, a capacitance strain sensor 10 includes opposed parallel plates 12, 14 separated by a pair of spacers 16, 18, which collectively define an air gap 20. Or, as shown in FIG. 1B, a capacitance strain sensor 22 may include a single spacer 18 to separate the opposed plates 12, 14, which may be preferred depending on the type of forces being sensed or required mounting configuration. As a compressive load is applied to the sensor, as shown, the separation between the opposed plates changes (e.g., narrows as shown in FIGS. 1A and 1B when subjected to a compressive load), thus causing a change in capacitance. In particular, capacitance, C, of a parallel plate capacitor can be characterized as being proportional to A×K/h where A is the plate area, K is the dielectric constant and “h” is the separation between the plates. As a result, the capacitance can be varied by changing the plate area, A, or the gap, h. The electrical properties of the materials used to form the sensor are generally unimportant, so the capacitance strain gauge materials can be chosen to meet the mechanical requirements of the particular application. Therefore, such sensors are useful in those instances where a more rugged sensor is needed, providing a significant advantage over resistance strain gauges. However, as discussed below, such sensors have drawbacks of their own.
Another type of sensor uses birefringence to detect strain forces. More particularly, such sensors operate on the principal that electromagnetic field waves in a deformed material can be subdivided into an amount of the field along one linear axis, and an amount of the field along another linear axis that is orthogonal to the first axis. The resulting two components of the electromagnetic wave are referred to as the perpendicular and parallel polarizations and thus these sensors are often called polarization sensors. Such polarization sensors comprise a sub-class of intensity sensing, whereby the sensor manipulates (in this case, rotates) the optical field's polarization state. In one implementation, the polarization sensor includes an optical analyzer to determine the amount of rotation that the field has undergone. For example, as shown in FIG. 2, a pressure sensor 30 utilizes the polarization effect on light intensity. Sensor 30 employs a photo-elastic material 32 that is sandwiched between a pair of parallel polarizer 34, 36. A compression plate 38 is disposed intermediate the polarizers, generally orthogonal thereto and acts as an interface to the object being measured (not shown), transferring forces acting on the object being measured to photo-elastic element 32. As pressure is applied to plate 38, plate 38 compresses the compliant photo-elastic element 32, thus altering the polarization of the light passing therethrough, and thus allowing a sensor 30 to quantify the amount of strain force based on changes in light intensity caused by polarization.
More particularly, in operation, input light provided by a fiber optic 42 impinges upon first polarizer 34 and the polarized light is transmitted to photo-elastic element 32. Element 32 then rotates the polarization state as the external pressure changes. The polarized light then impinges upon second polarizer 36 (which, when used in this configuration, is called an analyzer) designed to transmit light of only a certain polarization rotation to an output fiber optic 44. With no pressure applied, there is no polarization rotation, and hence the maximum field intensity emerges from the analyzer. As the pressure increases, more and more polarization rotation occurs, thus causing a decrease in the intensity of the light emerging from the analyzer. This intensity is quantified in conventional fashion to provide a measure of the strain force.
Although useful for certain applications, each of the above-described sensors has inherent problems. Resistive strain sensors require relatively complex measurement equipment (e.g., a wheatstone bridge), and can have less than ideal robustness. Moreover, resistive strain gauges dissipate a significant amount of heat, thus making their implementation impractical for many applications contemplated by the present invention including, for example, when the sensor is embedded with the object being sensed. Conventional capacitance strain gauges, although more robust than resistance strain gauges, are limited by the range of forces they can sense. For instance, measuring shear forces with a conventional capacitance sensor is difficult. Moreover, conventional air-gap capacitance strain sensors are not sufficiently sensitive for the applications contemplated by the present invention and, in any event, are vulnerable to overload in the presence of large forces, thus further limiting their application.
Piezoelectric sensors tend to be expensive and they are limited by the types of specialized materials that exhibit piezoelectric characteristics. Further, such piezoelectric-based devices are limited in their range of applications due to their relatively poor mechanical properties which limit the strain forces they can sense. Moreover, piezoelectric-based devices are typically made of brittle materials, have low material tensile strength and are operable only over a limited temperature range. Piezo-resistant wire sensors, on the other hand, typically require extremely sure mechanical contact, which can severely limit their use in many applications where a strong mechanical contact is difficult to establish and maintain. Known polarization sensors, such as that described in conjunction with FIG. 2, typically require that a substantial number of components (including polarizers, a light source, an interface, etc.) must be mechanically coupled to the object being sensed, thus adding to system cost and compromising overall robustness of the sensor. Moreover, because such sensors operate based on properties of light, their operational frequency range is limited.
In view of the above, the field of strain sensors was in need of a sensor design that employs a minimum of relatively inexpensive materials that exhibit superior mechanical properties, yet allow ready implementation in a variety of environments. Such a sensor should also minimize heat dissipation and not be limited in its range of applications. The sensor should be able to detect shear and normal forces acting on the object being sensed, and preferably provide a direct measurement without requiring any mechanical interface between the sensor and the object. Ideally, also, the sensor design should allow “tuning” capabilities according to particular sensing parameters, such as sensitivity and the like.